In order to display a high quality image on a large screen display constructed of a plurality of display elements, such as LEDs (light emitting diodes), which are arranged therein, the brightnesses of the display elements need to be uniform. However, variations occur in the brightnesses of the display elements due to a difference in the characteristics of each of the display elements. To solve this problem, a driving signal for driving each of the display elements is adjusted to make the brightnesses of the display elements uniform. In order to determine the degree of this adjustment, a measuring technique of detecting the brightness of each display element is required. Conventionally, as a brightness measurement method for use in display devices, there is a technique of capturing an image of a display screen to measure the brightness of each pixel on the captured image (for example, refer to patent reference 1).